Biomacromolecules 2001, 2, 824-826
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Synthesis of Acetylated Konjac Glucomannan and Effect of Degree of Acetylation on Water Absorbency Balint Koroskenyi and Stephen P. McCarthy* Biodegradable Polymer Research Center, Department of Plastics Engineering, University of Massachusetts Lowell, One University Avenue, Lowell, Massachusetts 01854 Received January 18, 2001; Revised Manuscript Received April 9, 2001
Konjac glucomannan was acetylated with acetic anhydride under different conditions to reduce the unusually high water absorbency of native konjac. The dependence of the degree of substitution (DS) on the reaction conditions and the influence of the DS on the water absorbency were investigated. The most efficient method for the acetylation was refluxing konjac in acetic anhydride in the presence of sodium hydroxide catalyst. The water absorbency rapidly decreased with increasing DS. Fully acetylated product was obtained within 12 h, which exhibited 1.0 g/g water absorbency vs the 105.4 g/g absorbency of native konjac. Because of the exponential decrease of water absorbency with increasing DS, a relatively small DS is sufficient to significantly suppress the absorption of water. Introduction Konjac flour is the generic name of the powdered tuber from Amorphophallus konjac, and occurs in a high molecular weight (in excess of 1 000 000) and is a polysaccharide classified as a glucomannan. Chemically it consists of mannose and glucose in a molar ratio of 1.6:1, respectively, with a β 1-4 linkage.1-3 The native polysaccharide contains an acetyl group per every 12 or 18 repeating units. Konjac glucomannan is known to reduce the cholesterol level via an interaction between cholesterol and the natural polysaccharide, thus preventing hypercholesterolemia. A number of studies have been carried out on the hypocholesterolemic and hypolipidemic properties of konjac.4-11 Due to this valuable characteristic, konjac is a potential drug candidate for the reduction of blood cholesterol levels and hence for the prevention of coronary heart disease.12 The advantages of konjac over current drugs are its biodegradability, biocompatibility, and, most importantly, the fact that it can be administered orally. Unfortunately, however, native konjac exhibits unusually high water absorbency, near 100 g of water/g of konjac. This prevents konjac from being used as a drug, since it absorbs water in the gastro-intestinal tract, causing diarrhea. The high water absorbency is due to the ability of the hydroxyl groups to form strong hydrogen bonds with water and trap the molecules due to structural characteristics. Blocking the hydroxyl groups by chemical modifications should theoretically reduce the water absorbency, preferably to an acceptable level. Presumably, the simplest way of blocking, as well as biologically the most friendly, is acetylation. Therefore, the acetylation of konjac was carried out and the water absorbency was measured. Acetylation of polysaccharides is a well-known reaction. A number of methods have been reported. Heterogeneous * Corresponding author. E-mail:
[email protected].
acetylation of cellulose is usually carried out in an aromatic hydrocarbon with acetic anhydride using a strong acid as catalyst.13,14 Homogeneous acetylation of cellulose has been carried out with both acidic and basic catalysts.15,16 Starch has been acetylated under similar conditions.17,18 Since our objective was to prepare acetylated konjac for drug research, the above procedures were modified to eliminate the use of toxic organic solvents. Experimental Section Konjac glucomannan was a donation of Multi-Ring Health Products, Ltd., Beihai, China. The molecular weight was between 1 000 000 and 1 100 000, and the moisture content was about 10%. A round-bottom three-neck flask equipped with a mechanical stirrer, a reflux condenser, and a thermometer was charged with 5.0 g of konjac, 50 mL of acetic anhydride, and 50% sodium hydroxide (NaOH) solution. The mixture was stirred and heated at the desired temperature for a predetermined period of time. The reaction conditions are shown in Table 1. The mixture was then allowed to cool, washed with water, precipitated with ethanol or methanol, filtered, thoroughly washed with alcohol, and finally dried in a vacuum. The degree of substitution (DS) was determined by titration according to Laignel et al.18 The following formula was used % acetyl )
(Va - Vb)NHClMacetyl × 100 ms
where Va ) volume of hydrochloric acid consumed for the blank in liters, Vb ) volume of hydrochloric acid consumed for the sample in liters, NHCl ) normality of the hydrochloric acid, Macetyl ) 43, and ms ) weight of the sample in grams.
10.1021/bm010014c CCC: $20.00 © 2001 American Chemical Society Published on Web 06/06/2001
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Synthesis of Acetylated Konjac Glucomannan Table 1. Reaction Conditions and DS Values of Acetylated Konjac Samples Ac2O, mL 10 150 50 a
50% NaOH, mL 1 1
temp, °C
time, h
DSa
refluxb
4 4 4
1.19 0.37 1.8
70 reflux
The DS of native starch is approximately 0.05-0.08. b ∼120 °C.
Figure 2. FT-IR spectra of konjac acetylated in the presence of 5 mL 50% NaOH solution: (a) konjac, (b) 0 h, (c) 2 h, (d) 6 h, (e) 6 h + 6 h, and (f) 12 h.
Figure 1. Degree of substitution vs reaction time at reflux temperature. Catalyst: (b) 1 mL; (9) 5 mL. [(0) Konjac acetate obtained by refluxing for 6 h was purified and subjected to acetylation for another 6 h.] The reaction time is measured from the time when the reaction mixture started refluxing.
The DS was calculated as follows: DS )
162 × % acetyl 4300 - 42 × % acetyl
The FT-IR spectra were recorded as KBr pellets using a Perkin-Elmer 1720X instrument. Water absorbency was determined according to the following procedure. Approximately 0.1 g of dry sample was placed into a 50 mL centrifuge tube. After adding ∼30 mL of deionized water, the mixture was allowed to stand for at least 1 h. The mixture was then centrifuged at 3000g for 30 min. The supernatant was removed and the remainder was weighed. The weight difference between the two measurements was taken as the weight of the absorbed water. Results and Discussion Acetylation of konjac was carried out in refluxing acetic anhydride with sodium acetate as catalyst. Because of the insolubility of konjac, the reaction was heterogeneous. The added sodium hydroxide presumably reacted with some of the anhydride to form the catalyst. The reaction conditions and the results are shown in Table 1 and in Figure 1. Although a significant amount of substitution took place in the absence of catalyst, the DS was considerably higher when catalyst was used. A larger amount of catalyst further increased the extent of the reaction. Temperature has a dramatic effect on the reaction rate. There is a 5-fold increase in the DS at reflux temperature (∼120 °C) compared to 70 °C. The DS value is plotted against time for the samples refluxed in the presence of NaOH solution in Figure 1. The
plot clearly shows the acceleration of the reaction by the increase in the amount of catalyst. A somewhat lower DS value was obtained at 12 h than with shorter reaction times. This can be due to experimental error. However, a completely acetylated product can be obtained in about 6 h. The results indicate an approximately 10% error in the determination of the degree of substitution. The DS of the starting material, native konjac glucomannan, was taken as 0, although there is some acetylation present, but the DS is very low (0.050.08, not measured). The IR spectra of acetylated konjac samples are shown in Figure 2. It is evident that at higher DS values the hydroxyl stretching band around 3500 cm-1 becomes smaller due to substitution. At the same time, the carbonyl stretching band at 1752 cm-1 resulting from the acetate ester groups becomes larger. Concomitant with the increase of the carbonyl stretching band, the C-C(dO)-O stretching band at 1238 cm-1 characteristic of acetates also becomes stronger. This provides further evidence for the increase in the substitution. Fully acetylated polysaccharides are usually soluble in certain organic solvents, such as chloroform, due to the increased hydrophobic character. Fully acetylated konjac did not dissolve in chloroform, although its behavior was entirely different from konjac acetates of smaller DS. Great swelling was observed, and the product floated in the solvent. The water absorbency dramatically decreased with increasing DS as shown in Figure 3. At a DS of 1.0, the water absorbency already drops to 10% of the original value. The nearly exponential decrease suggests that a relatively small degree of acetylation is enough to produce a nutraceutical or pharmaceutical product from konjac with more favorable properties. Conclusions Konjac acetate can be conveniently prepared in a heterogeneous reaction with acetic anhydride with or without NaOH catalyst. The reaction rate can be increased by increasing the catalyst concentration or the temperature. Fully acetylated konjac can be obtained in several hours. Acetylation is an excellent method to reduce the water absorbency
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Biomacromolecules, Vol. 2, No. 3, 2001
Koroskenyi and McCarthy
(Technology for a Sustainable Environment), is gratefully acknowledged. References and Notes
Figure 3. Water absorbency as a function of DS of konjac acetate. The DS of native konjac was taken as 0, although the actual value is probably between 0.05 and 0.08 (not measured) according to the small amount of acetyl groups (1/12-1/18 of repeating units) present.
of konjac. The latter drops to 10% of the original absorbency at a DS of 1.0. This technique is particularly expedient for applications in the field of nutraceuticals and pharmaceuticals, where the lowest possible water absorption and the use of nontoxic reagents are desirable. Acknowledgment. Financial support from the EPA/NSF National Center for Environmental Research Science to Achieve Results (STAR), Program Grant No. R826117
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